1. Field of the Invention
The invention relates to a method for creating low pressure in a brake activation device of a motor vehicle brake system.
2. Description of the Related Art
The invention relates to a method for creating low pressure in a brake activation device of a motor vehicle brake system, comprising a pneumatic brake force booster the interior of which is divided into at least one low-pressure chamber and one working chamber, a brake master cylinder and a pneumatic motor pump unit for creating the low pressure in the low-pressure chamber, which motor pump unit includes a low-pressure pump and a motor driving the low-pressure pump, a low-pressure level in the low-pressure chamber or a pressure difference between the low-pressure chamber and the working chamber being detected by means of a sensor and the motor pump unit being switched on by an electronic control unit if the pressure falls below a first predefined, lower low-pressure level in the low-pressure chamber and being switched off if the pressure reaches a second predefined, upper low-pressure level.
A method of this type is known, for example, from DE 199 29 880 A1, the low-pressure pump described therein being provided as a low pressure auxiliary pump which is connected when needed to the internal combustion engine serving as the main low pressure supplier, depending on operating states of the latter.
However, through the increased use of regenerative brake systems based on a brake force booster which can be activated in a driver-independent manner, which systems are used, for example, in hybrid vehicles with an electric motor and an internal combustion engine, operating states occur in these vehicles in which the internal combustion engine does not supply any low pressure for the brake activation device.
Furthermore, in particular in pure electric vehicles the required pressure must be generated solely by means of such a low-pressure pump.
Activation of the motor pump unit is usually effected when the pressure falls below a first predefined, lower low-pressure level (approximately 0.6 bar low pressure, that is, 0.4 bar absolute) in the low-pressure chamber. If a second predefined, upper low-pressure level (approximately 0.8 bar low pressure, that is, 0.2 bar absolute) is reached, the motor pump unit is switched off in order to reduce the load on the motor pump unit and therefore to prolong its service life.
However, tests have shown that when the low-pressure pump is used as the sole low pressure supplier the so-called overload point of the brake force booster, at which the maximum boost of the activation force is present and a further pressure build-up is possible only through a rise in the input force equal to the output force, can be very much lower in operation through the reduction in volume of the low-pressure chamber than with an internal combustion engine as the low pressure supplier.